In this example, the frequency of the second wave is slightly higher
than the first. The two frequencies are close enough so
that they form wave packets,
or beats.
The speed of the wave packets is called
the group
velocity. Note that the
group velocity is a bit slower than the
phase
velocity of the individual waves; that is, the packets move
slower than the single-frequency waves. This is typical for
normal dispersion, where the speed of waves decreases as frequency increases.